<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            width: 400px;
            height: 500px;
            background: red;
            border-top: 1px red solid;
        }

        .box div {
            margin-top: 50px;
            width: 100px;
            height: 100px;
            background: blue;
        }
    </style>
</head>

<body>
    <!-- maring塌陷 -->
    <div class="box">

        <div></div>
    </div>

</body>

</html>